In general, an induction motor includes a stator having a drive or excitation coil assembly, and a rotor having shorted coil assembly with a plurality of pole pairs. Typically, the stator is stationary while the rotor is rotatable with respect to the stator and is coupled to an output shaft for the motor. The motor generates torque due to the interaction between the stator magnetic field and the rotor magnetic field. The magnetic field of the rotor is induced from the stator by rotating the stator field at a somewhat different rate than the rotor rate. The difference frequency between the shaft (times the number of pole pairs of the motor) and the stator field frequency is known as the slip frequency. This slip frequency is seen by the shorted turns on the rotor. It is well known that the slip frequency strongly influences the basic machine characteristics such as torque constant and efficiency.
In the prior art, control of the induction motor is generally achieved by employing a slip control loop, that is, a feedback loop around an induction motor that slaves the stator excitation frequency to be controlled in order to establish frequency difference above or below the shaft or rotor rotation frequency (times the number of motor pole pairs).
One conventional induction motor servo implementation linearly varies motor slip frequency, and non-linearly varies motor current in order to hold the motor flux constant. While such a system is typically known as a "constant flux" implementation, as usually implemented this description is accurate only under steady state conditions. The conventional configuration is based on the principle that a constant machine air gap flux causes torque to be an approximately linear function of slip frequency. While controllers based on these principles do display relatively good linearity, they are substantially limited in terms of dynamic performance. In particular, a significant lag occurs at low slip speeds. Moreover, the large signal performance also tends to be limited.
In the prior art U.S. Pat. No. 3,824,437 and variations disclosed in related U.S. Pat. Nos. 3,805,135 and 3,796,935, a form of induction servo controller is disclosed with relatively good dynamics. In these patents, the voltage across the machine terminals is sensed and used in the weighting of machine excitations. In effect, the stator excitation is controlled by a feedback system that forces current phase to be controlled by the machine voltage or flux. However, the networks required to perform this feedback function are relatively complex, and have limitations based on cost and reliability considerations.
It is an object of the present invention to provide an improved controller for an induction machine.
Another object is to provide an improved controller for an induction machine characterized by a linear response to a relatively large torque and speed range for the machine.
Yet another object is to provide an improved controller for an induction machine which provides a high speed response over a relatively full torque and speed range for the machine.